Glioblastoma oncometry.

A prospective study of 20 patients was conducted to determine changes in the computed tomography appearance of glioblastomas seen at the completion of radiation therapy. An interval CT was obtained after 4000 to 4500 cGy to the whole brain and was compared to a similar baseline study. The tumor volume increased in twelve patients by 13 to 878% (mean 126%) and decreased in seven by 13 to 73% (mean 37%). It remained unchanged in one patient. A broadening or thinning of the enhancing rim frequently accompanied the increased or decreased tumor, respectively. Volume change immediately after whole brain radiotherapy was no prognostic indicator. The volume increase seen in 60% of the patients had implications for treatment planning of the boost field. It translated into a potential field size increase of up to 5.6 cm (mean 3.5 cm) and could contribute to a geographic miss. It is concluded that following whole brain radiation therapy, a repeat CT scan or magnetic resonance imaging, depending on the initial exam, is necessary for optimal planning of the reduced radiation field.